"People were still walking around, going upstairs to the buffet."
More than half of the national cases are linked to a 61-year-old woman known as "Patient 31".
The virus has been fatal in 2 percent of reported cases.
Samsung's factory in Gumi makes high-end phones.
A sharp fall in new deaths and cases of the coronavirus has been reported in China.
Zhao Yu admits her family wants her to stop, but she insists on doing her part.
"So, I'm finally in a room where I'll spend the next two weeks."
Some have tested positive for coronavirus on return to their home countries.
"We can avert any serious crisis, but if we squander the opportunity then we will have a serious problem."